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Rectangular PatcHow to measure, or know whether or not a printed antenna - RF Cafe Forums

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jinhao
 Post subject: How to measure, or know whether or not a printed antenna
Posted: Wed Apr 25, 2007 12:03 pm 
 
Lieutenant
User avatar

Joined: Tue Apr 24, 2007 1:10 pm
Posts: 2
is well / poor matched? since a printed antenna is part of the circuit board, it seems that we are not capable to measure VSWR.
This is a big problem for me.
Thank you very much.


 
   
 
nubbage
 Post subject:
Posted: Thu Apr 26, 2007 3:14 am 
 
General
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Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Do not despair, there may be a solution to this.
If you have access to the length of strip-line feeding the antenna element, it may be possible to fabricate a small directional sensor. This comprises a lambda/4 length of microstrip with a schottky diode at one end and a 100 ohm chip resistor at the other end, connected to ground. The diode has a bypass chip capacitor to ground on the dc side of the diode. With the diode oriented towards the antenna end the rectified energy is mostly due to, and proportional to, forward power. Reversing the orientation results in picking up and rectifying mainly power reflected back from the antenna. The ratio of the 2 readings is a very approximate measure of the VSWR.


 
   
 
rftroubleshooter
 Post subject:
Posted: Wed May 16, 2007 4:32 pm 
 
Captain
 

Joined: Wed May 16, 2007 4:16 pm
Posts: 5
If it were me, I'd spend the $1500 and rent a vector network analyzer. It will allow you to visualize what is going on with the impedence; you get a picture of the complex impedence, not just the VSWR.

This post has a brief method you could use to measure your PCB antenna using a VNA:



- Steve Montgomery
The RF Troubleshooter
www.rftroubleshooter.com




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